With reference to FIG. 1, a hydraulic brake actuating system 10 used in automotive vehicles typically includes a master cylinder 14 defining a cylinder bore 15 in which a primary piston 16 is translatably positioned. Ports in the cylinder bore fluidly connect the master cylinder 14 to hydraulic lines (not shown) of the vehicle's brake foundation system. The cylinder bore and the hydraulic lines are supplied with hydraulic brake fluid from one or more brake fluid reservoirs (not shown) fluidly coupled to the master cylinder 14. As is known in the art, the axial movement of the primary piston in the cylinder bore against the brake fluid in the master cylinder causes a pressure increase that is transmitted from the master cylinder through the brake lines to brake calipers or wheel cylinders at the wheels of the vehicle. The brake calipers or wheel cylinders are configured to urge brake pads against braking surfaces, i.e., drums, rotors, etc., of the wheel assemblies with a magnitude of force that corresponds to the position of the piston in the cylinder bore of the master cylinder 14.
A brake pedal 22 is used to control the movement of the piston assembly in the cylinder bore. In most hydraulic brake systems, a brake booster, such as brake booster 18, is coupled between the brake pedal and the master cylinder to translate the force applied to the brake pedal by the driver to an amplified force which is in turn applied to the primary piston of the master cylinder. As depicted in FIG. 1, a brake booster 18 typically comprises a booster housing 20 defining a bore 24 in which a power piston assembly 28 is translatably disposed. The power piston assembly 28 includes an input shaft 30 arranged in an input bore portion 25 of the power piston assembly 28, and an actuator pin 23 that extends axially from the input shaft 30 into a guide bore portion 32 of the power piston assembly 28. The brake booster 18 also includes a pedal rod 34 having a distal end 40 located in the guide bore portion 32 adjacent to the actuator pin 23 and a proximal end 31 coupled to the brake pedal 22. The distal end of the pedal rod 34 is approximately axially aligned with the guide bore 32. When the brake pedal 22 is pressed, the distal end of the pedal rod 34 is moved axially toward and into contact with a socket 36 at the end of the guide bore 32 that facilitates axial alignment between the distal end of the pedal rod 34 and the actuator pin 23.
The booster housing 20 includes an inlet port 35 through which a fluid (other than the brake fluid) is introduced into the housing 20, and an outlet port 37 through which the fluid is allowed to escape the housing 20. A normally open valve 38, also referred to as a throttle valve, is located within the power piston assembly 28 and between the inlet 35 and outlet ports 37 of the booster housing 20. During operation, the fluid enters the booster housing 20 via the inlet port 35, flows through the throttle valve 38 of the power piston assembly 28, and exits the housing 20 via the outlet port 37. To actuate the brakes, a force is applied to the brake pedal 22 by the driver that moves the distal end of the pedal rod 34 axially in the bore 32 which in turn causes the actuator pin 23 to move the input shaft 30 axially in the input bore 25. This movement closes the throttle valve 38 changing the degree of restriction of the flow of fluid through the housing 20. An increase in flow restriction results in a pressure increase on the input side of the power piston 28, pushing the power piston 28 toward the master cylinder primary piston resulting in an amplified force being applied to the master cylinder primary piston to actuate the brakes.
One difficulty faced in the operation of a hydraulic brake system as described above is retaining the distal end of the pedal rod within the interior walls of the guide bore 32. In previously known hydraulic brake systems, the distal end portion 40 of the pedal rod 34 has a hemispherical shape. A rubber grommet 44 is positioned around the shaft 29 of the pedal rod 34 adjacent to the distal end portion 40 to serve as a retaining collar that is configured to retain the hemispherical distal end of the pedal rod 34 in the guide bore 32. The outside diameter of the rubber grommet 44 resides in a cylindrical recess, which enables the grommet to move axially and also prevents the grommet 44 and pedal rod 34 from being pulled out of the power piston assembly 28. While effective, over time, the properties of the rubber material of the grommet, such as the hardness and/or elasticity, may change in a manner that adversely affects the ability of the grommet to retain the pedal rod in the guide bore 32.
In addition, during use, the pedal rod 34 is allowed some limited pivotal movement with respect to the axis of the guide bore 32. Using a rubber grommet to retain the distal end portion 40 of the pedal rod in the guide bore 32 as described above results in a change in the pivot axis of the rod as the rod is moved axially from a brakes released position to a brakes fully applied position. In the released position, the rubber grommet approximately centers the pedal rod on the axis of the guide bore, and pivoting occurs approximately about the center of the grommet. As the pedal rod travels towards the fully applied position, the hemispherical end of the pedal rod is forced into the mating hemispherical socket which brings the hemispherical end nearer to the center line of the actuator pin 23. Pivoting then occurs very close to the center of the hemisphere. Such variation in pivot location, and the friction resulting from forcing the hemispherical end into the mating socket introduces inconsistencies into the performance of the brake booster.